<!DOCTYPE html>
<html lang="zh-CN">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      .outer {
        width: 1000px;
        height: 100px;
        border: 1px solid black;
      }
      .inner {
        width: 100px;
        height: 100px;
        background-color: orange;
        /* transition复合属性：transition-duration transiton-property transition-delay transition-timing-function  */
        transition: 1s all 0.5s linear;
      }
      .outer:hover .inner {
        width: 1000px;
      }
    </style>
  </head>
  <body>
    <div class="outer">
      <div class="inner"></div>
    </div>
  </body>
</html>
